Spinal fusion typically involves the removal of damaged disc material between two adjacent vertebrae and the subsequent insertion of one or more interbody devices into the emptied disc space, either using an anterior or a posterior approach. In order to ensure primary stability, the surgeon usually adopts a fixation system that is anchored to the spine by means of orthopedic screws implanted into the pedicles of the vertebrae that are to be fused together. The single screws are connected together by means of rigid or semi-rigid rods, which are conveniently housed within a transversal hole provided in the screw head.
Since the FDA approval of pedicle screws, approximately 200,000 instrumented fusions occur each year in the US. There is very limited tolerance between the pedicle screw and the nerve root with the inferomedial wall of the pedicle (approx 1-2 mm). Current minimally invasive techniques increase risk of malposition. The pedicle screw may be inserted off center, such as, for example, too medial, which may impinge on the associated nerve root causing pain. This requires a repositioning of the screw. However, even after repositioning there may be an effect on the pedicle wall, which can still cause nerve root irritation. Such procedures are also susceptible to loosening of the screw.